This invention relates to an improved hydrolysis and condensation process for preparing cyclic siloxane compounds. More particularly this invention relates to a process for preparing cyclic siloxanes by hydrolysis and condensation of a hydrolyzable organosilicon compound, the improvement which comprises increasing the yield of said cyclic siloxanes by hydrolyzing and condensing said organosilicon compound in the presence of a cationic surface active agent, said agent being essentially soluble only in the aqueous phase of said process.
The hydrolysis and condensation of hydrolyzable organosilicon compounds, e.g. dichlorodimethylsilane to produce cyclic siloxane compounds, e.g. dimethylsiloxane trimers, tetramers, pentamers, etc., is a conventional process that is well known in the art, as seen e.g. by U.S. Pat. No. 2,905,703 and German Pat. No. 888,851. Heretofore, it has been found that cyclic siloxane product yields of about 50 percent can be obtained by carrying out the hydrolysis and condensation process at low temperatures and employing a strong mineral acid, e.g. HCl, H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4, along with large quantities of water. Higher cyclic product yields are obviously desirable for in addition to the obvious reasons for obtaining high yields, the formation of high concentrations of cyclic siloxanes will give the manufacturer a greater processing latitude. For example, following hydrolysis and condensation it is necessary to separate the siloxane product phase and aqueous phase. As the viscosity of the siloxane product phase increases, it becomes increasingly difficult to obtain a clean separation. An increased cyclic siloxane product content decreases the viscosity of the siloxane product phase, thereby allowing easier separation from the aqueous phase. Of course, as is understood, the siloxane product phase in addition to the desired cyclic siloxanes, also contains unreacted hydrolyzable and hydroxy end-blocked linear siloxanes. Such hydrolyzable linear siloxanes are generally neutralized with basic water solutions or repeated washings which must also be separated from the cyclic siloxanes product. Because such neutralization can lead to additional viscosity increases due to condensation reactions this second separation is generally more difficult than the first, hence the need for a low viscosity cyclic siloxane product is obvious. Further, in certain production operations low yields of dimethyl cyclic siloxane products are often depolymerized to yield additional cyclic siloxanes; a higher concentration of initial cyclic products may negate the need for depolymerization in some applications.
It is known that cyclic siloxane product yields can be increased by the use of water-miscible solvents. However, large amounts of solvents are normally required and such reduces the pot yield of cyclic product in proportion to the amount of solvent employed. Moreover, the preferred solvents, e.g. para-dioxane and tetrahydrofuran, are generally soluble in both the aqueous and siloxane phases of the process and any solvent present in the cyclic siloxane product must be removed when pure siloxanes are desired thereby requiring additional procedural steps and adding to the expense of the process.
It has now been discovered that high cyclic siloxane product yields can be obtained which can easily be separated from the aqueous phase by employing the improved hydrolysis and condensation process of this invention.